Large-Eddy Simulation of Boundary-Layer Transition over a Zigzag Trip

Wall-resolved large-eddy simulations (WR-LES) are conducted to study transitional and turbulent boundary-layer properties and flow structures downstream of a zigzag trip on the low-Reynolds-number SD7032 airfoil at Re-c=1x10(5) and 2x10(5) and angle of attack =0 deg. Without a zigzag trip, the airfoil features a prominent laminar separation bubble on the suction side. First, a verification and validation study of the LES method is conducted for the zigzag trip at a roughness-height-based Reynolds number Re-k=240. The boundary-layer velocity profiles and the integral parameters are compared with experimental data acquired using a long-distance mu-particle image velocimetry setup in a wind tunnel. In the LES, if the zigzag tape is thick enough to trigger transition just behind the tape, the transition to turbulence is generally quicker than what was observed in the experiment. Although the general phenomenology and the development of the turbulent boundary layer after transition are consistent with the experimental data, there remain some differences in the exact length of the transitional region and also in the associated airfoil drag. A zigzag tape with Rek & AP;200 or thicker induces transition quickly behind the trip through a shedding process of the shear layer created by the tape. The boundary layer behind the zigzag tape features prominent three-dimensional vortical structures emanating from the zigzags that persist over long chordwise distances and well into the turbulent boundary layer. Nevertheless, in a spanwise-averaged sense, the integral thicknesses of the newly created turbulent boundary layer are actually quite similar to a two-dimensional boundary-layer theory solution, given that the tape has a suitable thickness. In contrast, an overly thick tape will generate a thicker turbulent boundary layer, where the increased displacement and momentum loss are sustained and also contribute to total drag.